1. Field of the Invention
This application in general refers to the use of certain compositions, generally referred to as phase change materials (PCM's) which change their phase, generally between solid and liquid phases, and thereby store heat energy during such change. More particularly, it refers to such PCM's which have the effect of storing coolness, because they melt and freeze at a temperature below room temperature.
2. The Prior Art
It has long been recognized that the heat generated by solar energy during daytime hours can be stored by various means to provide heat during those times when the heat from the sun is insufficient to provide energy in requisite amounts. More recently, there has been an awareness of the need for storing not heat, but what might be perceived as coolness, i.e., heat energy at temperatures substantially lower than body temperature or room temperature. Perhaps the most simple example of such coolness storage is a block of ice that releases its coolness--actually, it takes up heat from the surrounding ambience--as it melts. The present invention is specifically concerned with the storage of coolness.
The use of coolness storage has become of increasing importance in residential, but most particularly in commercial applications. Almost irrespective of outside temperatures, it has become necessary to provide some means for dissipating heat in a commercial structure. Where a building houses a manufacturing operation, obviously there is heat generated due to the operation of the equipment. Even where the structure is an office building, there is heat generated by electric lights, by office or kitchen equipment, and simply by the presence of human beings. As a consequence, the storage of heat for use during cooler periods has not found widespread application, except for specialized uses such as in solar hot water heaters or in greenhouse applications, where the practical, economic effectiveness of such devices has yet to be proved.
Coolness storage has been attempted on a commercial scale, although successes have been few, and some failures notable. In practice, coolness storage has generally been limited to such storage utilizing chilled water or ice. Chilled water storage has the immediate and blatant limitation that chilling water at off-peak times and then utilizing it during peak hours when coolness is required, takes advantage only of the specific heat of the water, rather than its heat of fusion. As an example, since the heat of fusion of water is 80 calories per gram, utilizing only the specific heat of water would require a volume many times that which could be gained by utilizing the heat of fusion of water-to-ice storage. If the water used for its specific heat is to have the same volume as water in which coolness is stored by heat of fusion, it would have to fall 80.degree. C. in order to store as much coolness. Consequently, the space occupied by chilled water storage is a major drawback in the practical application of that procedure to the storage of coolness.
There are other serious problems in using chilled water to store coolness in addition to the requirement of great volumes of water to be chilled. When a large volume of water has been chilled to the required temperature, say 48.degree. F., for circulation throughout a building, the return flow of warmed water mixes with the chilled water, thereby raising the temperature of the chilled water to an unacceptable level. Thus, chilled water storage is not believed to be an acceptable means of coolness storage which will gain widespread acceptance.
Recognizing the infirmities of chilled water storage, ice storage has long been considered as a possible alternative. While the problem of space attendant in chilled water storage is not present with ice storage, other problems are immediate. The difference in densities between ice and water presents a problem. Freezing a container full of water will exert a pressure on the container, and if the container is made of metal, the corrosiveness of the water will become evident, particularly in the presence of oxygen. Further, in a retrofit application where the chiller normally used to cool the building is already in place, that chiller will not be able to produce temperatures low enough to form ice, e.g., about 28.degree. F. Instead, the chiller unit must be modified, in effect to become a low-temperature refrigeration system. Yet the quantum of energy necessary to achieve temperatures of 28.degree. F. are substantially greater than those necessary to chill water to 50.degree. F., at about which temperature most commercial, chilled water, air conditioning systems operate. Because the energy usage is not a straight-line function of the temperature to be achieved, it is uneconomical to make ice, use it to chill water to 32.degree. F., then blend that water with tap water to achieve a 50.degree. F. mixture that will be suitable for use in cooling structures.
Still another problem of ice storage systems is the fact that are usually constructed above grade, where the requisite space is often simply not available. Thus, the problems inherent in coolness storage systems that utilize ice as the storage means have prevented widespread use of that type of system as well.
That type of coolness storage system which appears to hold the greatest promise for commercial utilization is one which is well known, but which has utterly failed to achieve the success for which it seemed imminently destined for the past several decades. That type of system is the use of salt hydrates and additives and modifiers thereto to form eutectoid compositions that have their freezing (and melting) point controlled to a predetermined value. Such eutectoid salt mixtures are generally known as phase change materials. They have primarily been used to store heat, but also have been known to store cooling capacity.
To a large extent the use of PCMs to store coolness may be traced to patents and publications of Dr. Maria Telkes, a pioneer still active in the field. As early as 1954, in U.S. Pat. No. 2,677,664, Dr. Telkes described the use of borax (sodium tetraborate decahydrate) as a nucleating agent to seed salt hydrates so that supercooling is avoided and crystallization will be commenced at the eutectic point. The use of borax as a nucleating agent for sodium sulfate decahydrate was specifically disclosed by Dr. Telkes.
More recently, in U.S. Pat. No. 3,986,969, Dr. Telkes referred to the problem of manufacturing a homogeneous mixture of sodium sulfate decahydrate with other ingredients to form a eutectoid composition. She reviewed many acknowledged thickening agents and proposed a specific clay as an improved thixotropic agent. In that patent Dr. Telkes also set forth some known salts used to form eutectic mixtures with sodium sulfate decahydrate and to control a less-than-room temperature melting point thereof.
According to Dr. Telkes, it would appear that in order to achieve a sodium sulfate-based eutectoid having a freezing point somewhat below 50.degree. F., it is necessary to use ammonium chloride. In this manner, freezing points of about 47.degree.-48.degree. F. can be achieved. One difficulty with the use of ammonium chloride is that it is a relatively expensive compound compared to other salts that make up a eutectoid mixture, e.g., sodium sulfate, potassium chloride, etc. As a result, its use has deterred commercialization of PCM's as a coolness storage medium.
In a so-called Final Report entitled, Bulk Storage of PCM, dated December 1977 to June 1980, which report was prepared for the Department of Energy, Office of Solar Applications, by Calmac Mfg. Corp., a formulation was proposed to achieve a "plateau" at 45.degree.-48.degree. F. The eutectic disclosed in that report, which was based on work of Dr. Telkes, is essentially one wherein there is a molar ratio of potassium chloride to sodium sulfate to ammonium chloride of 1:4:8. While such a plateau, or range of solidification and melting points, is satisfactory for most PCM storage installations, it will be apparent that by using so much ammonium chloride, not only is the expense of the PCM substantially increased, but on a weight basis less storage capacity is being achieved. It has been determined that the heat of fusion of the 1:4:8 composition proposed by Dr. Telkes is about 38 BTU per pound of PCM. This heat of fusion, while certainly satisfactory, is one that is subject to definite improvement.
Another difficulty with the use of large amounts of ammonium chloride, in addition to the lowering of the heat of fusion of the PCM, is the expense, as mentioned. Because there is no known large-scale use of ammonium chloride, and because ammonium chloride is not generally a by-product of large-scale production of other chemicals, its price is such that its use in quantities proposed for eutectoid mixtures, i.e., in a mole ratio of 2:1 to Glauber's salt (sodium sulfate decahydrate) in order to obtain freezing points below 50.degree. F., has made the obtaining of such freezing points not impossible, but economically undesirable. Economy of storage is vital to the acceptance of coolness storage systems employing PCM's, rather than simply the effectiveness of such systems. Exemplarily, even the most effective PCM will prove a failure in the marketplace if the payback period for its installation is ten years or more. Coolness storage through the use of PCM's has remained a laboratory curiosity for the past forty years rather than a commercial success not because of possible unreliability, but because it can cost so much that the period during which the investment can be recovered through time-of-use rates is just too long.